1. The Field of the Invention
The present invention relates to semiconductor devices and methods for their construction. More particularly, the present invention relates to capacitor design and cell isolation methods used to reduce the surface area occupied by a DRAM cell. More specifically, the present invention merges capacitor design and cell isolation methods by using existing isolation trench sidewalls to form a DRAM capacitor thus increasing DRAM cell density by at least about two fold over currently fabricated DRAM cells.
2. Background Art
Various DRAM capacitor designs have been employed to reduce the surface area occupied by a single DRAM cell. Early DRAM designs employed flat horizontal capacitor plates. Later designs, intended to conserve chip surface area, employed trenches or fin structures to form narrow dimension capacitors with some vertical contribution to the capacitor plate surface area.
In addition to the shape and size of the capacitor plates, the type of cell isolation contributes to the overall DRAM cell size. Traditionally, field oxide produced by the Local Oxidation of Silicon process (LOCOS) was used as cell isolation. Unfortunately, a field oxide must cover a fairly wide area in order to effectively isolate adjacent cells. Further, it is difficult to control the growth of field oxide. Therefore, field oxide often occupies a significant amount of the chip surface area.
More recently, trench isolation has been employed. This involves etching a narrow isolation trench around the active areas (cells) on the chip. The isolation trenches are then filled with oxide or other dielectric to effectively isolate adjacent active areas from one another. While trench isolation requires more process steps than field oxide isolation, trench isolation can be made much narrower than field oxide isolation. Therefore, DRAMs employing trench isolation can be packed more densely than DRAMs employing field oxide (LOCOS) isolation.
In the continuing quest for higher density DRAMs, improved structures employing trench isolation are still needed.